(1) Studies of whole body calcium dynamics have been continued using lactating women with themselves as controls at a time at least one year from the lactation period. Simulations of the time period required to determine the first of presumably several bone exchange parameters suggests that studies be carried out for at least six weeks rather than the three used previously. (2) Determination of fractional absorption using the dual tracer technique and monitoring the tracers in urine has been studied in infants (6), adult women (14) and two siblings with end organ vitamin D resistance. Total absorption in the first two groups was virtually the same, but there is a strong suggestion of age dependent slowing in the rate of absorption. The fractional absorption of the vitamin D resistant siblings was very low (approximately 10%), unresponsive to treatment with vitamin D or 1,25-OH2-D, and was invariant under the clinical treatment of repletion via intravenous calcium.